1. The Field of the Invention
The present invention relates to multi-shaft auger systems and processes for mixing soil with a chemical hardener in situ in order to form soilcrete columns, walls, and piles. More particularly, the present invention permits improved soil/hardener mixing over a larger area in less time than the prior art soil mixing techniques.
2. The Prior Art
For a number of years, multi-shaft auger machines have been used in Japan to construct concrete-like columns in the ground without having to excavate the soil. These columns are sometimes referred to as "soilcrete" columns, because the soil is mixed with a cement hardener in situ. Upon hardening, the soilcrete columns possess characteristics of concrete columns, but they are constructed without the expense and time-consuming processes of removing and replacing the soil with concrete.
The soilcrete columns have been arranged in a variety of patterns depending on the desired application. Soilcrete columns are used to improve the load bearing capacity of soft soils, such as sandy or soft clay soils. The columns are formed deep in the ground and form a solid base or "foundation" for anchoring or supporting surface construction on such soft soils.
In other cases, the soilcrete columns have been overlapped to form boundary walls, structural retaining walls, low to medium capacity soil-mixed caissons, and piles which act as a base for construction.
To produce soilcrete columns, a multi-shaft auger machine bores holes in the ground and simultaneously mixes the soil with a chemical hardening material pumped from the surface through the auger shaft to the end of the auger. Multiple columns are prepared while the soil-cement mixture is still soft to form continuous walls or geometric patterns within the soil depending on the purpose of the soilcrete columns.
Because the soil is mixed in situ and because the soilcrete wall is formed in a single process step, the construction period is shorter than for other construction methods. Obviously, the costs of forming soilcrete columns are less than traditional methods requiring excavation of the soil in order to form concrete pillars or walls. In addition, because the soil is not removed from the ground, there is comparatively little material produced by such in situ processes that must be disposed of during the course of construction.
Historically, a modified earth digging auger machine is used in the formation of in situ soilcrete columns. The boring and mixing operations are performed by multi-shaft drive units in order to make the process more efficient. The shafts typically contain soil mixing paddles and augers which horizontally and vertically mix the soil with the hardening material, thereby producing a column having a homogeneous mixture of the soil and the hardener.
As ground penetration occurs, the chemical hardener slurry is injected into the soil through the end of the hollow stemmed augers. The augers penetrate and break loose the soil and lift the soil to mixing paddles which blend the slurry and the soil. As the auger continues to advance downwardly through the soil, the soil and slurry are remixed by additional augers and paddles attached to the shaft.
Two-shaft and three-shaft auger machines have been used to construct soilcrete columns. Three-shaft machines were first developed for constructing continuous wall structures, such as diaphragm walls and retaining walls. The three shafts mutually overlap so that each individual shaft produces a column which is physically linked and overlapped with the adjacent columns. The net result is a homogeneous and continuous soilcrete wall. However, the amount of overlap is limited in order to avoid interference between adjacent augers.
Unfortunately, three-shaft auger machines require a tremendous amount of power to drive each shaft. It will be appreciated that individual shaft power requirements are proportional to the cross-sectional area of the column. Thus, as the column column radius increases, the shaft's power requirements increase by an amount proportional to the square of the radius increase. As a result, three-shaft auger machines have not been particularly successful in constructing large diameter continuous walls.
Auger machines with more than three shafts have also been used in Japan to construct continuous walls in situ, but the walls were relatively thin compared to those constructed with a typical three-shaft auger machine. Generally, as the number of shafts to be driven increases, the auger diameter of each shaft decreases.
It has been found that a two-shaft auger machine can produce larger diameter columns with less power than a three-shaft machine. Furthermore, if the total power previously used to drive three shafts is applied to drive just two shafts, a larger diameter column may be produced in unusually hard and/or rocky soil. Therefore, two-shaft auger machines have been used to produce larger diameter columns than a three-shaft machine. Nevertheless, while a two-shaft machine can produce such columns in harder soil and yet consume less power than a three-shaft auger machine, the efficiency of the overall process is less because substantially more strokes up and down through the soil are necessary in order to construct a wall of a given length.
In actual use, the prior art has used the two-shaft auger machine such that each borehole is "double bored" in order to insure that adjacent columns overlap and are continuous. While this produces a very homogeneous soilcrete structure, it is very time consuming and uses more of the chemical hardener.
The alternative is to use a three-shaft auger machine, but this results in a wall having weak areas because the soil/hardener mixture may not be homogeneously mixed. In order to assure a homogeneous mixture within each column and so that adjacent columns are substantially comparable in strength, density, and integrity, each column would have to be "triple bored," which is an uneconomical process.
Despite its apparent advantages, the two-shaft auger machine does possess some disadvantages. As mentioned above, the two-shaft auger machine is less economical in use because it takes longer to form a column or wall over a given area vis-a-vis using a three-shaft auger machine. Moreover, the two-shaft auger machine can result in the use of more chemical hardener than a three-shaft auger machine. Since material costs are a significant portion of the total costs of the prior art processes, this further makes the two-shaft auger machine less economical to use.
Moreover, due to mechanical limitations, it has been found that a two-shaft auger machine cannot have the same degree of shaft overlap as a three-shaft machine. In fact, the shaft overlap on a two-shaft machine is so slight that the two shaft machine forms in situ columns essentially tangent to each adjacent column. As a result, a two-shaft auger machine is not particularly useful for preparing continuous wall formations which are to function as a boundary or cut off wall.
Thus, two-shaft auger machines have been most useful in the prior art in soil or foundation improvement projects where longitudinal support is needed. For instance, the columns are arranged in various geometric patterns such as triangles, squares, or hexagons to improve the load-bearing capacity of the soil.
A two-shaft auger machine has also been used on other large scale soil improvement projects. For example, continuous, large-area columns may be constructed by overlapping and offsetting individual soilcrete columns. However, because a two-shaft auger machine does not create columns with substantial overlap, numerous voids or interstitial spaces of unmixed soil are formed when a two-shaft auger machine is used to construct large-area solid columns.
These voids and interstitial spaces have been eliminated only through extensive overlap and offset of the soilcrete columns or through additional boring if no offset or overlap is used. However, such procedures result in increased time and expense which are important factors in any construction project.
From the foregoing, it will be appreciated that what is needed in the art are apparatus and methods for forming soilcrete columns, walls, and piles which permit increased effective overlap between adjacent columns without sacrificing column diameter or requiring larger power equipment.
It would be another advancement in the art to provide apparatus and methods for forming larger soilcrete columns with an effective overlap between the columns which may be constructed in series in order to form continuous soilcrete walls or piles.
It would be a further advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles with increased effective overlap between adjacent columns in order to provide improved soil stabilization or foundational support capabilities.
Additionally, it would be a significant advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles which permit efficient construction of continuous large-area columns without creating interstitial voids and without requiring substantial column overlap and offset.
It would be yet another advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles which minimize the amount of chemical hardener required, thereby reducing construction costs.
It would be a further advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles which require fewer penetrating strokes than conventional apparatus and methods, thereby providing increased construction time efficiency.
It would be an additional advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles with increased versatility, wherein the same equipment may be used to construct different types of walls, columns, and piles.
It would be another advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles which have a homogeneous blend of soil and chemical hardener without increasing the construction time, the overall cost, and the amount of materials consumed.
It would be a further advancement in the art to provide apparatus and methods for forming soilcrete columns, walls, and piles wherein adjacent columns possess substantially equal strength, density, and integrity properties.
The foregoing, and other features and objects of the present invention are realized in the improved multi-shaft auger apparatus and methods which are disclosed and claimed herein.